Over the years, I've been surprised at the damage done to some container grown specimens that have had their roots exposed to prolonged temperatures in the 20's. Two blueberry plants had lost 1/2 their tops under such circumstances. I'm now much more careful regarding the matter and always dig in any pots I'm wanting to overwinter outside.

Terry

Put them somewhere just above freezing, so they thaw fairly slowly (a fast thaw could do even more damage)

Resin

Some quite hardy plants have root death in containers in the 20s. Youngest roots out by container walls most vulnerable. Only a short spell below min. temperature sufficient to kill them.

We're only down to 29 or so. Is that a problem?

No, it is not. Your plants risk nothing. I never lost anything when the temps did not get below -6°C. Not even zone 9 conifers such as Pinus canariensis or Araucaria bidwillii. Best you can do is to protect the foliage from direct sunlight. About roots, Pinaceae are usually hardier than Cupressaceae.

A lot depends on the size of the containers as well. I grow a good portion of my dwarf conifer collection in containers, most in the 16" diameter size and up, and they have withstood some pretty low temperatures without harm. Most are 3-5 years old and have experienced several cold spells into the low 20's. FWIW, I have never offered winter protection for any of these containerized plants and even so-called tender conifers (ie. Cupressus macrocarpa 'Wilma Goldcrest') have come through cold winters without damage. I do make sure containers have been well-hydrated before and anticipated cold snap.